Photomultiplier



LAhmmLu 455-619 AU 233 EX FIPBlO OR 3,390,272

June 25, 1968 M. B. FISHER 3,390,272 PHOTOMULTIPLIER Filed March 31,1965 2 Sheets-Sheet 1 I I7 saeb INVENTOR. MAHLON B. FISHER ATTORNEYPOWER SOURCE LIGHT SOU RCE June 25, 1968 M. B. FISHER PHOTOMULTIPLIER 2Sheets-Sheet 2 Filed March 31, 1965 womnom PIG:

INVENTOR.

MAHLON B. FISHER BY ATTORNEY United States Patent 3,390,272PHOTOMULTIPLIER Mahlon B. Fisher, Monte Serene, Calif., assignor toSylvania Electric Products Inc., a corporation of Delaware Filed Mar.31, 1965, Ser. No. 444,241 17 Claims. (Cl. 250-199) ABSTRACT OF THEDISCLOSURE This photomultiplier comprises an electron gun and helicalslow wave structure supported in axial alignment and located in an axialmagnetic field. The cathode and dynode electrodes of the electron guncomprise axially spaced truncated cones having external electronemissive surfaces. -A hollow focus-accelerating electrode is radiallyspaced from and is supported coaxially around and substantiallycoextensive with the cathode and dynodes. The focus-acceleratingelectrode also has a truncated conical shape and has an opening thereinthrough which light is passed to the photosensitive surface of thecathode.

This invention relates to photomultipliers and more particularly tomeans for detecting and amplifying microwave modulation of opticalwavelength signals.

One method of detecting a modulated light beam is th use ofphotoelectron emission from a photosensitive cathode illuminated by themodulated light beam. Since the current generated by the photoelectronemission process is very small, it is desirable to amplify the currentto a sufficient magnitude to be more readily useful. It is particularlydesirable to amplify the photoelectron current such that the shot noisecaused by irregularities in the emission of electrons from the cathodeexceeds the thermal noise produced by random motion of electrons in theoutput coupling mechanism caused by thermal action. Conventionalphotomultiplier tubes such as described in volume NS-l'l, IEEETransactions on Nuclear Science, June 1964, are available formultiplying photoelectron current modulated by low frequency signals tosuch a level. These devices are not suitable, however, for amplifyingphotoelectron current modulated at microwave frequencies because of therelatively large spread or variation in transit time of electrons suchas when traveling from one end of the tube to the other which ischaracteristic of multipliers. Photomultipliers proposed for amplifyingsignals generated by optical wavelength radiation modulated at microwavefrequencies are described in Dynamic Crossed Field Electron Multiplier('DCFEM), by O. L. Gaddy and -D. F. Halshauser in Proceedings of theIRE," volume 50, No. 2, page 207, February 1962; High SpeedPhotomultipliers, by R. C. Miller and N. C. Wittwer, Bell TelephoneLaboratories, Inc., in IEEE International Convention Record, volume 13,part 5, page 7, 1965; and LASECONS: Microwave Phototubes WithTransmission Photocathodes," by D. J. Blattner et al., in IEEEInternational Convention Record, volume 11, part 3, page 79, 1963.Particularly, the latter photomultiplier has limited current carryingcapability, is fragile and has relatively short life. The other twophotomultipliers require relatively large and heavy magnets and have alow equivalent resistance such that many stages of multiplication arerequired to provide a specified signal level.

An object of this invention is the provision of a photomultipliercapable of operating with large beam currents while having a longer lifethan has been heretofore achieved.

Another object is the provision of a, microwave photomultiplieremploying a small lightweight magnetic focusing structure.

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Another object is the provision of a photomultiplier requiring fewerstages than conventional microwave photomultipliers to obtain the samedegree of amplification.

Another object is the provision of a microwave photomultiplier havinglow transit time spread.

Another object is the provision of a microwave photomultiplier having ahigh current density electron beamforming multiplier structure capableof operating with a helical slow wave structure.

In accordance with a preferred embodiment of this invention, an opticalwavelength signal modulated at microwave frequencies is incident on thephotosensitive cathode surface of an electron gun in an axial magneticfield. The electron gun comprises an elongated cathode shaped like acone axially aligned with and spaced from a plurality of axially spacedsimilarly shaped dynodes. A hollow focus-accelerating electrode alsohaving the shape of a truncated cone is supported coaxially around andsubstantially coextensive with and radially spaced from the cathode anddynodes. A photosensitive emission layer on the external cathode surfaceis located adjacent to an opening in the focus-accelerating electrodethrough which the modulated light beam is directed toward the cathode.'Each electron emitted by the cathode travels in a cycloidal path andgenerates a plurality of secondary electrons upon impact with theadjacent dynode. The secondary electrons also follow a cycloidal pathand successively strike adja-' cent dynodes to generate successivelygreater numbers of secondary electrons at each dynode. The beam ofsecondary electrons from the last dynode traverses an adjacently locatedhelical slow wave structure and terminates at a collector electrode atthe opposite end of the slow wave structure. The modulation signal onthe electron beam is transferred to a traveling wave on the slow wavestructure. The amplified signal is coupled from the traveling wave onthe helix by well known coupling techniques.

This invention and these and other of its objects will be more fullyunderstood from the following description of a preferred embodimentthereof, reference being bad to the accompanying drawings in which:

FIGURE 1 is a longitudinal cross sectional view of a traveling wavephotoelectron multiplier embodying this invention and employing asolenoid focusing structure;

FIGURE 2 is a modified form of the embodiment of FIGURE 1 wherein thephotoelectron multiplier employs a permanent magnet focusing structure;and

FIGURES 3 and 4 are modified forms of the electron beam formingmultiplier structure shown in FIGURES l and 2.

Referring to FIGURE 1, the photoelectron multiplier comprises aphototube 1, a solenoid 2 and output coupling means 3. Phototube 1 is anelectron tube comprising an evacuated vacuum envelope 4 enclosing anelectron gun or beam-forming multiplying structure 5, a slow wavestructure 6 and a collector electrode 7.

Electron gun 5 comprises a cathode 8, axially aligned dynodes 9a and 9b,a coaxial hollow focus-accelerating electrode 10 in the shape of atruncated cone and a ring electrode 11. Electrode 10 performs the dualfunction of electrostatically focusing electrons and accelerating theirmovement through the gun structure. Cathode 8 is symmetrical about itslongitudinal axis and comprises truncated-cone sections 8a and 8b. Thedynodes are also truncated-cone electrodes symmetrical about theirlongitudinal axes. The cathode and dynodes have a central openingextending the length of each electrode for receiving a support ormounting rod 12 made of electrically nonconductive material such asaluminum oxide. Cathode 8 and dynodes 9a and 9b are axially spaced fromeach other on and rigidly secured to rod 12 such as by brazing.Alternatively, the cathode and dynodes may be secured i l l to supportrod 12 by swedging tabs 8' and 9' into mounting recesses in the supportrod. The slope of the conical surfaces and the axial spacing of cathodesection 8b and the dynodes are such that a plane through thelongitudinal axes of the dynodes and section 8b of the cathode cutstheir outer surfaces in a straight line.

Dynodes 9a and 9b are preferably made of beryllium copper. The dynodesare perferably fired in an oxidizing atmosphere under a partial vacuumto form a secondary emission coating of beryllium oxide on the outersurface thereof. Cathode 8 is preferably a non-magnetic metal. The outersurface of conical cathode section 8b is coated with a photosensitiveelectron emitting material. When an S1 photosensitive surface such asdescribed in Methods of Experimental Physics, number 6, part B, page389, by A. H. Sommer and W. E. Spicer, Academic Press, 1959, isemployed, the cathode is made of solid silver. The photosensitivesurface is preferably formed by oxidizing the silver under a partialvacuum and then subjecting the cathode to a cessium atmosphere untilmaximum cathode emission is obtained.

The wall of the focus-accelerating electrode 10 has an opening 10thereon adjacent conical cathode section 8b. Electrode 10 and ringelectrode 11 are physically connected such as by glass beads 13 embeddedin mounting pins 14 which are welded to the electrodes. Ring electrode11 is electrically connected to slow wave structure 6 and lead pin 15such as by a choke helix 16. Electrode 10 is secured in the vacuumenvelope by tabs 17 which are welded to the electrode and pins 18.

Alignment rod 12 is secured to a metal mounting plate 20 by a nut 21.Mounting plate 20 is secured to a header 22 by tabs 23, which are weldedto the mount plate and header pins 24. Rod 12 has several holes (notshown) extending axially therethrough. These holes contain wires forelectrically connecting the cathode and dynodes to separate header pins24. The header assembly comprises plate 20, rod 12, cathode 8 anddynodes 9. When header 22 is sealed to vacuum envelope 4, the headerassembly is positioned such that electrode 10 is coaxial with andsubstantiallycoextensive with the cathode and dynodes. The overall axiallength of electrode 10 is slightly greater than the overall length ofthe cathode and dynodes.

Slow wave structure 6 is an eflicient circuit for coupling energy fromthe electron beam since it has a high equivalent resistance. The slowwave structure is preferably a helix of the type employed inconventional traveling wave tube amplifiers. One end of helix 6 iswelded to pin 15 adjacent the end of the vacuum envelope containingelectron gun 5. The other end of the helix is welded to output pin 15'adjacent collector electrode 7.

Output coupling means 3 comprises a cavity 26 surrounding pin 15. Theinner conductor of a coaxial transmission line 27 is connected to pin15. The other end of the coaxial transmission line is connected toconnector 28.

Solenoid 2 provides a uniform unidirectional axial magnetic field overthe length of the phototube. When a lightweight structure is required,the photomultiplier employing the permanent magnet structure illustratedin FIGURE 2 may be employed to provide this magnetic field. Thisalternate structure comprises a cylindrical magnet 31, a magnetic polepiece 32 and a periodic permanent magnet structure 33. Cylindricalmagnet 31 has a transverse opening 31' in its wall aligned with opening10' in electrode 10 and cathode section '8b. Magnet 31 is coaxial withand substantially coextensive with electron gun 5 for providing auniform axial magnetic field over the combined length of cathode 8 anddynodes 9. Pole piece 32 prevents the periodically varying magneticfield of magnet structure 33 at helix 6 from adversely affecting theunidirectional axial magnetic field at the cathode and dynodes. Outputcoupling means 3 preferably comprises a coupling helix 26'.

The dynodes, electrodes and helix of the phototube are electricallyconnected through a connector 34, see FIG- URE l, to a source 35 of DCpotential. Cathode 8 is maintained at a reference potential. Dynode 9ais maintained at a positive potential with respect to the cathode. Eachdynode is maintained at a somewhat higher potential than that of theadjacent dynode in the direction of the cathode. Focus-acceleratingelectrode 10 is maintained at a much higher positive potential thaneither the dynodes or the cathode. The electric potential on ringelectrode 11 and helix 6 is maintained at the synchronous velocity of atraveling wave associated with the helix. Collector electrode 7 ispreferably maintained at a somewhat more positive potential than thehelix. In certain applications, helix 6 and focus-accelerating electrode10 may be operated at the same potential by electrically connecting pinsand 18, respectively. The cone angles of cathode 8, dynodes 9a and 9band focus-accelerating electrode 10 and the electrode potentials areadjusted to provide between both the axially aligned cathode and dynodesand the coaxial conical electrode 10 an electric field having axial andradial components.

In operation, an amplitude-modulated light beam 36 from light source 37is reflected by optical mirror 38 through aperture 10' of electrode 10to illuminate a spot on the photosensitive surface of conical cathodesection 8b. Modulation of light beam 36 is detected by thephotosensitive cathode which generates an electron beam whose electrondensity varies at the modulation rate of the light beam. Primaryelectrons are emitted perpendicular to the surface of cathode 8. Theseelectrons are simultaneously accelerated radially towardfocus-accelerating electrode 10 and axially toward the dynodes by theelectric field between these electrodes. The axial magnetic fieldimparts to the electrons a circular motion about the axis of thecathode. Thus, the space charge waves of the electron beam comprise bothtransverse and longitudinal wave components. The primary electrons movein a cycloidal or spiral path about the axis of the tube and strike theouter surface of dynode 9a. The transit time spread of the electron beamis reduced through acceleration of electrons by the high voltagefocus-accelerating electrode 10 While concurrently maintaining thesecondary emissive dynode.9 at a relatively low voltage. Electronsstriking dynode 9a cause a greater number of secondary electrons to beemitted by the dynode. The secondary electrons from dynode 9a alsofollow a cycloidal path about the axis of the tube, strike dynode 9b,and similarly cause an even greater number of secondary electrons to beemitted from that electrode. More electrons are emitted by each dynodethan strike it. Thus, the density of the electron beam emitted bycathode 8 is increased or multiplied by operation of dynodes 9. Althoughonly two dynodes are illustrated here, more may be employed as requiredfor a higher multiplication factor. Electrons emitted from dynode 9bdescribe a helical path about the axis of the tube within and adjacenthelix 6 and are collected by electrode 7.

Modulation of the electron beam is in the form of space charge waves.These space charge waves comprise both transverse and longitudinalmodulation components as a result of the helical trajectories of theelectrons in the gun region. The slow wave structure supports atraveling electromagnetic wave having a fixed phase velocitysynchronized with the slow space charge waves and energy is interchangedbetween the synchronized longitudinal and traveling waves. Themodulation signal is extracted from the traveling wave by cavity coupler26 and provided at connector 28 for connection to external equipment.

The power output P of a multiplier is where i is the modulationcomponent of the photocurrent and R is the equivalent resistance of themultiplier. The equivalent resistance of the DCFEM and Miller-Wittwerphotomultiplier is about 50 ohms as opposed to an equivalent resistanceof about 1000 times that value for a photomultiplier incorporating thisinvention. Thus, a very large photocurrent or multiplication is requiredto obtain a specified signal level from the DCFEM and Miller- Wittwerphotomultipliers. In contrast, the same output may be obtained with amultiplier incorporating this invention with a much smaller photocurrentor multiplication because of the very high equivalent resistance of theoutput circuit. For example, a multiplier having a 50 ohm equivalentresistance has an output power of where M is multiplication. Amultiplier incorporating this invention has an output power of EquatingEquations 2 and 3, it is seen that a photomultiplier incorporating thisinvention requires only 7% of the multiplication of the multiplierhaving a 50 ohm equivalent resistance to provide the same output power.Thus, the photomultiplier incorporating this invention has a longer lifesince current at and heating of the last dynode are substantially less.If additional cooling of cathode 8 and dynodes 9 is required or desired,alignment rod 12 may be made of a heat conductive electricallynonconductive material such as beryllium oxide. Special precautions mustbe taken, however, in metallizing and firing this material.

Referring to the modified forms of electron gun 5 in FIGURES 3 and 4,cathode 8" and dynodes 9" may be elongated cylindrical electrodes. Thefocus-accelerating electrode may comprise a plurality of concentric ringelectrodes 41 (FIGURE 3) or truncated-cone electrodes 42 (FIGURE 4)which are axially spaced apart and supported coaxially of andsubstantially coextensive with the cathode and dynodes. The DC electricpotential applied to electrodes 41 or 42 are adjusted to providesubstantially the same electric field between both the cathode anddynodes and the focus-accelerating electrode as in the embodiment ofFIGURES 1 and 2.

By way of example, a traveling wave photomultiplier similar to theembodiment of FIGURE 1 was built and tested and had the followingparameters and operation:

Number of dynodes 3 Cone angles (included angle):

Cathode section 8b 424 Dynodes 9 424 Electrode 10 1048 Axial length:

Cathode section 8b inch 0.250 Dynodes 9 (each) do 0.118 Electrode 10..do 1.0 Inner diameter, minimum:

Electrode 10 inch 0.352 Outer diameter, minimum:

Cathode section 8b inch- 0.160 Helix:

Barrel fluted TPI 10.0 Wire diameter -inch 0.015 Inner diameter do 0.350Length do 8.0 Voltages:

Cathode 8 .-volts 0 Dynode 9a ..do 300 Dynode 9b -do 600 Dynode 9c do900 Electrode 10 do 3500 Electrode 11 do...... 3500 Helix 6 do 3500Collector 7 do 3560 Magnetic field gauss 1000 Modulation index "percent"20 6 Cathode 31 Light beam diameter ....inch -0.020 Modulated cathodephotocurrent amperes, average 0.2 10' Current from last dynode do--40x10- Equivalent resistance ohms 3 X 10* Power output ....d.b.m 35Bandwidth G c.p.s.-.. 1-3 Signal to noise ratio db 45 Although thisinvention is described in relation to a preferred embodiment thereof,variations and modifications will be apparent to those skilled in theart. For example, light beam 36 may comprise both a local oscil1atorlight signal and a modulated light beam in order to provide heterodyneoperation and an intermediate frequency signal from the cathode. Also,the structural rigidity of the tube may be increased by inserting intomounting rod 12 a reinforcing rod 44 which is secured to collectorelectrode 7 and extends axially down the length of helix 6. The scopeand breadth of this invention, is therefore to be determined from thefollowing claims rather than from the above detailed description of apreferred embodiment thereof.

What is claimed is:

1. A photoelectron gun comprising:

first electrode means comprising an elongated enclosure open at least atone end and having an axis,

a plurality of other elongated electrodes supported coaxially withinsaid first electrode means and being axially spaced from each other,

one of said other electrodes adjacent one. end of said first electrodemeans having an external photosensitive electron emissive surface,

the other of said other electrodes being dynode electrodes havingexternal electron emissive surfaces,

said first electrode means having an aperture for passing a beam oflight therethrough to the photosensitive surface of said one of saidother electrodes,

means for selectively electrically energizing said electrodes, and

means for generating a unidirectional axial magnetic field.

2. The photoelectron gun according to claim 1 wherein said otherelectrodes are truncated cones.

3. A photomultiplier comprising:

first electrode means comprising an elongated enclosure open at least atone end and having an axis,

a plurality of other elongated electrodes supported coaxially withinsaid first electrode means and axially spaced from each other,

one of said other electrodes adjacent one end of said first electrodemeans having an external photosensitive electron emissive surface,

the remainder of said other electrodes comprising dynode electrodeshaving electron emissive surfaces,

said first electrode means having an aperture for passing a beam oflight therethrough to the photosensitive surface of said one of saidother electrodes,

a collector electrode axially aligned with and spaced from said otherelectrodes opposite from said one of said other electrodes,

means for selectively electrically energizing said electrodes, and

means for establishing an axial magnetic field at said electrodes.

4. A photomultiplier comprising:

first electrode means comprising an elongated enclosure open at least atone end and having an axis,

a plurality of other elongated electrodes supported coaxially withinsaid first electrode means and axially spaced from each other,

one of said other electrodes adjacent one end of said first electrodemeans having an external photosensitive electron emissive surface,

8 the other of said other electrodes comprising dynode 8. Thephotoelectron gun according to claim 7 whereelectrodes having electronemissive surfaces, in said cathode is cylindrical. said first electrodemeans having a wall with an aper- 9. The photoelectron gun according toclaim 7 wherein ture for passing a beam of light therethrough to thesaid cathode is a truncated cone. photosensitive surface of said one ofsaid other elec- 10. The photoelectron gun according to claim 7wheretrodes, in said dynode is cylindrical. a collector electrodeaxially aligned with and spaced 11. The photoelectron gun according toclaim 7 wherefrom said other electrodes opposite from said one saiddynode is atruncated cone. of said other electrodes, 12. Thephotoelectron gun according to claim 7 wherea slow wave structureaxially aligned with and located m in said focus-accelerating electrodemeans comprises a between said other electrodes and said collectorelectruncated cone. trode, 13. A photoelectron gun comprising: means forselectively electrically energizing said eleca truncated cone cathodeelectrode having an elontrodes and said slow wave structure, and gatedexternal photosensitive electron-emissive surmeans for establishing anaxial magnetic field at said face,

electrodes and through said slow wave structure. a truncated cone dynodeelectrode having an external 5. The photomultiplier according to claim 4wherein electron emissive surface and being axially spaced said slowwave structure is ahelix. from said cathode and having an axiscoincident 6. A photomultiplier for amplifying a signal associated withthe axis of said cathode, and with a beam of electrons generated inresponse to a modufocus-accelerating electrode means comprising: latedlight beam, said photomultiplier comprising: a hollow truncated conesection spaced from, co-

first electrode means comprising an elongated open axial with andsubstantially coextensive with enclosure having an axis, said cathodeand dynode, the wall of said cone a plurality of other elongatedelectrodes supported section forming an opening therein adjacent saidcoaxially within said first electrode means and Photostmsitive Surfaceof Said cathode, and axially spaced from each other, a cylindrical ringaxially spaced from said cone one of said other electrodes adjacent oneend of said section and having an axis coincident with the firstelectrode means having an external photosensiaxis of said dynode. tiveele tr i iv f c 14. A traveling wave photoelectron multiplier for amtheother of said other electrodes comprising dynode plifying a signal on abeam of electrons generated in reelectrodes having electron emissivesurfaces, sponse to a modulated light beam, said multiplier comsaidfirst electrode means having a wall with an aperprising:

tu f passing th modulated light bea therean evacuated electricallynon-conductive enclosure conthrough to the photosensitive surface ofsaid one of taining: said other electrodes for generating a modulated anelongated cathode electrode that is symmetrical b a f el tron about itsaxis located at one end of said ena collector electrode axially alignedwith and spaced closure, said cathode having an elongated phofrom saidother electrodes opposite from said one tosensitive electron-emissivesurface, of said other electrodes, an elongated dynode electrode axiallyspaced from a'slow wave structure axially aligned with and located 40Said Cathode, Said dynode having an axis coincibetween said otherelectrodes and said collector dent with the axis of said cathode andbeing lectrod symmetrical about its axis, means for selectivelyelectrically energizing said elecfocus-accelerating electrode meanshaving a ho]- trodes and said slow wave structure for simullow elongatedsection symmetrical about its axis taneously accelerating the electronsradially toward and having an opening therein adjacent said said firstelectrode means and axially toward said photosensitive surface of saidcathode, said accollector electrode, celerating electrode being spacedfrom, coaxial means for establishing an axial magnetic field at saidwith and substantially coextensive with said electrodes and said slowwave structure whereby the cathode and said dynode, electrons move in aspiral path about the axis and a collector electrode located at theopposite end strike the one of said dynodes adjacent said one of of saidenclosure from said cathode and having said other electrodes porducingsecondary emission an axis coincident with the axis of said cathode, ofa larger number of electrons moving in a spiral and path and strikingsuccessive dynodes for generating a slow wave structure located betweenand axially more secondary electrons, the electrons from said alignedwith said focus-accelerating electrode dynode opposite said one of saidother electrodes means and Said colleCtOl' eltici-l'ode, Saidctraversing said slow wave structure for coupling f being Symmetricalabout an axis coincident energy to a traveling wave thereon, and withthe f of said e, means for coupling a signal from said slow wave meansfor focus ng a modulated light beam through mama the opening 1n saidfocus-accelerating electrode A photoelectron gun comprising: means ontosaid photosensitive cathode surface for producing electrons therefrom,

means for applying DC potentials to said electrodes and said slow wavestructure for establishing an axial and radial electric field betweensaid cathode, dynode and said focus-accelerating electrode means foraccelerating electrons radially from said cathode and an elongatedcathode electrode symmetrical about its axis, said cathode having anelongated external photosensitive electron-emissive surface,

an elongated dynode electrode axially spaced from said cathode, saiddynode having an axis coincident with the axis of sa d cat a beingSymmetrical about axially toward said collector and for focusing the itsaxis, and electrons,

focus-accelerating electrode means having a hollow means forestablishing a substantially axial magnetic elongated sectionsymmetrical about its axis and field over the length of said multiplierfor causing having an opening therein adjacent said photosensitheelectrons to spiral about the axis and strike said tive surface of saidcathode, said accelerating elecdynode to cause emission of secondaryelectrons trode being spaced from, coaxial with and substantherefrom,and for causing the secondary electrons tially coextensive with saidcathode and said dynode. 7-,; to traverse said slow wave structure tocouple energy to a traveling wave on said structure, said electronsbeing collected by said collector electrode, and

means for coupling a signal from said slow wave structure.

15. A traveling wave photoelectron multiplier for amplifying a signal ona beam of electrons generated in response to a modulated light beam,said multiplier comprising:

an evacuated electrically non-conducting enclosure containing:

a truncated cone cathode electrode having an elongated photosensitiveelectron-emissive surface, said cathode being located at one end of saidenclosure,

a truncated cone dynode electrode axially spaced from said cathode andhaving an axis coincident with the axis of said cathode,

focus-accelerating electrode means comprising:

a hollow truncated cone section spaced from, coaxial with andsubstantially coextensive with said cathode and dynode, the wall of saidcone section having an opening therein adjacent said photosensitivesurface of said cathode, and

a cylindrical ring axially spaced from said cone section and having anaxis coincident with the axis of said dynode,

a collector electrode located at the opposite end of said enclosure fromsaid cathode and having an axis coincident with the axis of saidcathode, and

a helix slow wave structure located between and axially aligned withsaid cylindrical ring and said collector electrode, said helix beingsymmetrical about an axis coincident with the axis of said cathode,

means for focusing a modulated light beam through the opening in saidfocusaccelerating electrode means onto said photosensitive cathodesurface for producing electrons comprising an electron beam having anassociated space charge wave varying at a rate proportional to themodulation of the light beam,

means for applying DC potentials to said electrodes and helix forestablishing axial and radial electric fields between said cathode,dynode and said focusaccelerating electrode means for accelerating electrons radially from said cathode and axially toward said collector andfor focusing the electrons,

means for establishing a substantially axial magnetic field over thelength of said multiplier for causing the electrons to spiral about theaxis and strike said dynode to cause emission of secondary electronstherefrom and for causing the secondary electrons to traverse said helixto couple energy to a traveling wave thereon, said electrons beingcollected by said collector electrode,

means for varying the DC potential applied to said helix forsynchronizing the velocity of the space charge wave of the electron beamwith the fixed phase velocity of the traveling wave on said helix, and

means for coupling a signal from said helix.

16. A photomultiplier comprising:

first tube-like electrode means having an axis,

a plurality of other tube-like electrodes supported co axially withinsaid first electrode means and axially spaced from each other,

one of said other electrodes adjacent one end of the first electrodemeans having an external photosensitive electron emissive surface,

said first electrode means having an aperture therein for passing a beamof light therethrough to the photosensitive surface of said one of thesaid one of the other electrodes, and

means for selectively electrically energizing said electrodes.

17. The photomultiplier according to claim 16 in combination with amicrowave slow wave circuit at the other end of the first electrodemeans and aligned with the axis thereof.

References Cited UNITED STATES PATENTS 2,125,750 8/1938 Ramberg 313-2,160,798 5/1939 Teal 313-405 3,154,748 10/1964 Javan et al. 315-35 X3,231,742 1/1966 Siegman 315-3.5 X 3,258,626 6/1966 Kino et al 3l5--3.6X

HERMAN KARL SAALBACH, Primary Examiner. S. CHATMON, JR., AssistantExaminer.

